In recent years, the use of ink jet printers in numerous applications has increased dramatically. Such printers make use of liquid-based inks which are jetted onto a receptor, typically a sheet of paper or film, to produce an image. By using four basic ink colors (cyan, magenta, yellow, and black) in various combinations and concentrations, virtually any color may be produced as part of the printed image. Additionally, ink jet technology is well-suited for high resolution graphic images, particularly those produced using electronic printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics and the like.
Many of the inks that have been used in the past with ink jet and other printers are primarily comprised of dyes contained within organic- or water-based carrier liquids. Although such inks may offer satisfactory performance in certain applications, the present trend is away from such systems, since such systems tend to produce images that lack the light stability and durability required for outdoor and similarly demanding applications.
In order to solve this problem, inks prepared by using pigments, instead of dyes, as colorants have been investigated.
Pigmented inks for ink jet applications have to meet several stringent requirements. For example, particle size has to be very small and particle size distribution has to be narrow in order to avoid pigment settling and nozzle plugging. The dispersion has to have good stability against agglomeration in order to avoid nozzle plugging and to have good shelf life. The ink has to have good thermal stability, particularly for application in thermal ink jet printers, in order to avoid "kogation".
Pigment dispersions are thermodynamically unstable. The suspended pigment particles tend to agglomerate due to attractive van der Waals forces. Since ink jet printers make use of very small jet nozzles (on the order of less than about 80 micrometers) to provide high resolution images, the resulting pigment agglomeration has a tendency to plug the printer heads. Additionally, in the case of thermal ink jet systems, such inks also suffer from the tendency of materials to settle onto, and coat, the heating elements inside the nozzles of the printer head. This causes a reduced thermal efficiency of the print head which results in the formation of smaller ink droplets, lower drop velocity, and lower image quality. This effect is commonly referred to as "kogation".
To overcome the problems described above, dispersants are typically employed to adsorb onto pigment surface to build a protective layer, (either electrostatically or stericly or a combination of both,) around each particle to counteract the attractive forces.
In one approach, as disclosed in U.S. Pat. Nos. 5,125,968 and 4,959,661, the dispersants are selected from surfactants which contain a hydrophilic portion as well as a hydrophobic portion.
In another approach, polymers having hydrophilic segments and hydrophobic segments have been used. Polymer dispersants having both random and block structures have been disclosed. Examples of these approaches are described in U.S. Pat. No. 4,597,794 and U.S. Pat. No. 5,085,698.
Other attempts involving the use of polymeric dispersants have been made as well. For example, water-soluble polymers such as styrene-acrylic acid copolymers have also been considered, yet these have yielded only partial success. In particular, such systems, although promising, have tended to produce non-uniform printed solid block images. The lack of uniformity in the printed image, which becomes more pronounced with prolonged printing, is caused by incomplete coverage of the receptor surface in the image area.
This problem, commonly referred to as "banding" results from progressively smaller projected ink drops over the course of a printing job. This effect is believed to be a result of kogation, caused by deposition of thermal insulating materials on the heating elements within the printing cartridge. As a result, heat transfer efficiency into the ink is decreased, thereby reducing the ability to produce properly sized ink bubbles needed for the printing process. Even if the deposited material is thermally conductive, it may still change the nucleation behavior on the heater surface during heating which also may adversely affect the bubble formation. Another problem which is often observed to be associated with polymeric dispersants is that the ink tends to solidify at a nozzle tip to cause obstruction.
Another critical requirement for a pigmented ink jet ink is long decap time so that crusting of the ink composition does not occur on the nozzle plate either during printing or when the printer is in the idle mode.
"Decap time" is defined as the length of time over which an ink remains fluid in the nozzle openings when exposed to air so that ink drops can be fired at their intended targets. Crusting of the nozzle plate will result in poor print quality or even worse may completely plug nozzles thereby causing total print failure. Humectants may be added to promote long decap times, but they must be carefully selected to neither cause flocculation of the pigmented ink dispersions nor promote kogation.
A third important property for pigmented inks is the ability to dry fast once ink drops are placed on the receptor to produce the intended image. Fast drying is important because it is one of the key factors that determines the printer productivity.
The requirement of long decap time and the requirement of fast printed image drying are often in conflict. For example, in order to overcome the crusting problem and improve decap time, a significant amount of a high boiling cosolvent such as ethylene glycol or diethylene glycol is normally employed in the ink composition to reduce the rate of evaporation. However, since these types of cosolvents tend to dramatically reduce the drying rate of the printed image that diminishes the requirement of fast printed image drying.